Method of chemically plating metals



METHOD OF QHEM-IQALLY PLATING METALS George Li Schnable, Lansdale, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation ofPenne sylvania N0 Drawing; Application March 21, 1956 Serial No. 5725828= 7 (Claims. (Cl.,l17--- 150) The present invention relates to the chemical plating ofv metal coatings onto articles containing certain other base metals; and, more particularly, the invention relates to a novel 'methodfor chemicallyplatingonto zinc, cadmium ormanganese or alloys thereof, lower melting metal coats ings whichare more resistant to corrosion and aremore readily soldered than the basemetal.

In many industries today, particularly in the manufacture of electronic equipment, it is desirable to provide a metal coating on articles of zinc, cadmium, manganesev or alloys of two or more of these which,is.1nor.e resistant to-corrosion, more readily soldered and melts. at; a., lower temperature than the base metal. Attempts to do this by immersing the base metal into a molten bath of the-coat ing inetal covered by a suitable flux results in coatings the thickness of which cannot be controlled. Moreover, thehigh surface tension of certain of metals in molten. form, inostnotably indium, makes. wetting of irregular surfaces on. the base metal difficult. Certainoi the lower, meltingrnetalscan also be electroplatedout or aqueous solutioiisfontcthe base metal. However, in this case the, deposits are crystalline, and have a relatively largesur; face area. Even with plating solutions, having good. throwing power irregular objects are unevenly. coated. In order to obtain smooth coatings, the electroplated 0b ject must subsequently be immersed in a hot flux tomelt. the deposit. Of course, When the base metal articleis, very, fine or complex, the making off the necessary elec: trical contacts for electroplating is difficultrendering electroplating impractical. Attemptsto chemically plate the lowr rnelting, metals onto the base metal out of; aqueous. 40 solution have failed inasmuch asthe deposit is. spongy and otherwise unsatisfactory, and, deposition in such; a system stops as. soon as an initial layer. of depositv is.v formed unless an extraneousreducing agent is added to he el lian.

ii'a c ject ft er iesen't inventionto PIG-7 method for providing metal coatings on, articles containing zinc, cadmium, manganeseor alloys. ttwo o more ofthese.

n. that, object of h pre nt inventioh. is to provide. a. noyel method for providing, by a chemical plating opera-. ti'oh, fsmooth, uniform coatings, onzinc, cadmium, man ganeseoralloys o'f two or more otthese, comPrising metal which is, rhQreeorrosion-resistant and morelea sily soldered alanine Still another objectofthe, present invention ,istoprovide a hovel-method fer torfrninghy chemical plating; smooth, dense, homogeneous and unifortn metal coatings of the type ir entioned above on bases of. Zinc, cadmium, manganese or alloysof two or more oi theseevenrwhen, the base metalpieseiits anigluy irregular or com lex. surface.

Other objects will b'ecjome apparent froma con ideratioh of, the followingspecification and the claims The method Ofjtllje pr sent inventt'" "coin mersing the article to be coated cor'nprising at of the metals selected frornthe group censisti cadmium andmanga'r'ies'ein a litfuid non aqueops bath comprising salt of inetaliother than the base "metal selected; andinore noble than the base metal, until the j 70 amou t o'fino'r'e noble in a nite States Patent the solidus point of the base metal but-above the liquidus pointoi the metal coating.

it has been found that simple immersion of zinc; cadmium or manganese, or alloys of two or xnore'oi these, in a hot, non-aqueous bath'coniprising salt of metal, other than: the particular base metal selected and more noble than the particular base metal, especially those selected from the group consisting of bismuth, indium, tin, lead-, thallium, cadmium and Zinc, at a temperature below the soliduspoint of the base metal results in chemical plating of the more noble, lower-melting metal onto the base in the form of a molten coating, when the temperature of the bath is above the liquidus point of the metal coating. In this way a coating of any desired thickness can be formed. Upon removal of the coated article from the bath, the molten metal rapidly solidifies to provide auniform coating of any desired thicknesswhich is dense, smooth and homogeneous. Moreover; inaddition to the high quality of the coating, the coating is uniform over irregular surfaces and even on inside, but exposed; sun faces.

The-present process, being a chemical plating procedure based. on the phenomenon of simple chemical displacement due to relativedifferences-in position in the electroinotive series, does not rely on any flow-of current, as in electroplating, nor on an extraneous reducing agent. There is no need to maintain electrical contact as in the case of electroplating so that the fineness or complexity of the parts is not material.

The-present IllEilI'OdrlS applicable to tlie coating of articles comprising at least one ofthe metals se-lected'ftoin the group consisting of'zit'ic, cadmium and manganese, that iszinc, CflCllTlllIIllOi manganese or alloys containing two or more of these. In such alloys the cornpositinn-v may vary widely. from those containing substantial amounts of each metal to those in which one of" the constituents is present in only a minute-amount, such as about 0.1%, byweight. Examples of such alloys are c-admium zinc alloys containingbetween about li.l 99L9 cadmium to 99.90.1% Zinc, by weight, the eutectic being- 17% zinc to 83% cadmium.

Such base metals are readily wettedby the molten metal, and the molten metal deposit adheres readily thereto. Any desired amount of metal can be deposited, from a verythinmolecular film to heavy "deposit's depend ing upon the results desired, the nature of the base and the amountot'. plating inetal in the bath. in fact if the bath containsian amount of plating inetal at -least chemically. equivalent to thebase metal, the base metal can be completely displaced by the more noble metali Thus platin'gcan be continued until all of the base met'al has been sodisplaced, .or, the procedure can be stopped'shorb of complete displacement; toprovide a coatingof any desired. thickness. The complete displacement of-bas'e metal is a practical consideration in many instances. For: example, where it is desired to apply a sma-ll dot. of indium to a germaniumbase, indinmbeing soft and'dif ficult tohandle as small pellets, a dot 'of zine 'or the like maybe applied to the base and'the zincthenxdisplaced by indium, in accordancewith: theppresent. procedure.

The base. metal is: soluble. in the plated metal forming, asthe metal coating; an'allo'y. in.moltentform"therewith;

It is: this feature that permits plating 'to eontiuue once-aw coatingpi's formed since the formationof th'e' alloy makes: base,;met-al available at the interface betwcenztlre coating and the bath. The; exact "composition. of the alloyformed may vary widely. depending; upon) the phase: diagram ofi-tlre-wystemand thetcmperaturemf: operas tion, the higher the; temperatur he more; solublettlre? base meta winthecoating, Th *0 entraticnof. 2 361.

e al.ixt.thes aflnamaylag-astlowassa bont ufiltat mies percent, n L g n H 1 The metal plated out must be more noble than thebase metal onto which it is deposited. Examples of metals that may be deposited on zinc, cadmium or manganese or alloys thereof in accordance with the present procedure are bismuth, indium, tin, thallium and lead. In accordance with the present procedure, zinc may also be deposited on manganese, cadmium may be deposited on manganese and cadmium may be deposited on zinc. Combinations of metal may be deposited by use of salts of two or more of the stated metals in the bath. As mentioned above, in any case the coating is actually an alloy, one constituent of which is the base metal and the other or others of which is the more noble metal or metals from the bath.

As stated, the procedure comprises simply immersing the base metal into a liquid, non-aqueous bath comprising salt of more noble metal to be deposited at a tem-' perature below the solidus point of the base metal but above the liquidus point of the metal coating formed. The metal to be deposited will exist, just prior to deposition, in solution in the bath in the form of ions. The bath may be a molten salt bath comprising molten salt of metal to be deposited or may be a solution of salt of metal to be deposited in a high boiling organic solvent therefor. In the case of the fused salt bath, the bath may consist substantially entirely of molten salt of the more noble metal. For example, a salt of indium, lead, thallium, bismuth or tin, or of cadmium when cadmium is to be deposited on manganese or zinc or of zinc when Zinc is to be deposited on manganese, which has a melting point below the solidus point of the base metal se lected, may be simply melted to form the bath. Of such salts the halides are especiallysuitable, particularly the chlorides and bromides. Examples of suitable salts, in this connection, are indium monochloride, stannous chloride, thallous chloride, plumbous bromide, bismuth chloride, cadmium chloride, Zinc chloride, and the like. It is preferred that the more noble metal be in a lower valent state in the bath. For example, preferably indium exists in the bath at a valence below three, that is, one and/or two, tin exists as stannous ions, lead as plumbous ions and thallium as thallous ions. The maintenance of the multivalent metals in a lower valent state will present no problems to those skilled in the art, and any one of several expedients serving to maintain the metal in a reduced ionic state may be employed. For example, the surface of the bath may be covered with a non-oxidizing, that is a neutral or reducing, atmosphere,

such as carbon dioxide, hydrogen, helium, argon, nitrogen, or the like. The multi-valent metal ions may also be prevented from oxidizing to the highest valent state by maintaining, in the bath, a body of the elemental metal as such.

In addition to salt of more noble metal, the fused salt bath may contain other salts. When the bath contains salts of metals it is not desired to deposit, the metals will, as a general rule, not be more noble than the base metal. For example, in the case of plating onto a base comprising zinc or cadmium, salt of the metal to be deposited may be mixed with a zinc salt, the zinc salt merely serving as diluent or solvent. The above-stated general rule is subject to some exception where the metal I of the diluent salt and the more noble metal are relatively far apart in the electromotive series in which case the most noble metal in solution will selectively plate out on the base even though the metal of the diluent salt is somewhat more noble than the base metal. For example, in depositing indium on manganese, the bath may also contain a zinc salt in which the indium alone will be deposited. Salts of the alkali metals, such as their halides, may be employed as diluents or solvents. Nonmetal salts, such as ammonium salts, may also be employed. Examples of fused salt mixtures serving merely as diluent or solvent are a combination of zinc chloride and ammonium chloride, especially in a 3 to 1 weight ratio (the eutectic melting at 180 C.); a mixture of lithium chloride and potassium chloride, especially in a 5 to 6 weight ratio (the eutectic melting at 352 C.); and the like.

The use of a diluent salt is advantageous in many instances to control the rate of deposition. The rate of chemical plating appears to be directly proportional to the concentration of ions of metal to be deposited. For example, Zinc immersed in molten stannous chloride at 800 C. reacts vigorously with heat evolution to form molten tin-zinc alloy. When, however, zinc metal is immersed in a molten mixture containing equal parts, by weight, of zinc chloride and stannous chloride, the reaction proceeds smoothly, coating the Zinc with molten tin-zinc alloy at the rate of about 0.08 inch per minute.

On the other hand, the bath may comprise a solution of salt of the more noble metal in a high boiling organic compound, or mixture of such compounds, which at the temperature of operation serves as a solvent for the salt and in which ionic conduction occurs at the temperature of operation. Such compounds are normally polar and will have a boiling point above the temperature of operation. Examples of polar organic compounds that may be employed are the polyhydric alcohols, such as glycerols, the glycols, for instance ethylene glycol, the polyethylene glycols, especially diethylene glycol, 1,2,4-butane triol, sorbitol, and the like; glycol derivatives, such as ethylene glycol monobutyl ether, and the like; high boiling amines, such as triethanolamine, N,N,N',N'-tetrakis (2-hydroxypropyl) ethylene diamine, and the like; amides such as acetamide; phenol; phthalic anhydride; benzoic acid; and the like. The organic material may be solid at ordinary temperatures so long as it is liquid at the temperature of operation, as is the case with many of those materials mentioned above.

The concentration of the metal, in ionic form, in the bath is relatively immaterial since the process is selective and quantitative. In other words, when only small amounts of metal are to be plated on a small article, a concentration of the plating metal as little as about 0.001 normal, at the temperature of operation, may be sufiicient. On the other hand, when the bath i made up entirely of salt of the plating metal, the metal, in ionic form, will make up 100 atomic percent of the cations of the bath. Advantageously, the concentration of plating metal in the bath is at least about 0.01 normal at the temperature of operation.

During operation, the bath will be at a temperature below the solidus point of the base metal, preferably at least 40 C. below, but above the liquidus point of the metal coating. Hence, the exact temperature of the bath will depend upon the system employed and particularly upon the metal coating formed. Since the metal coating formed is an alloy, the temperature of the bath will be above the liquidus point of that alloy although it may well be below the melting points of each of the individual metals. For example, when tin is chemically plated onto cadmium in accordance with the present process there is formed a tin-cadmium alloy which melts below 200 C. Although cadmium itself melts at 321 C. and tin itself melts at 232 C., the bath in this case may be operated at a temperature in the neighborhood of about 200 C. to form the molten alloy. Likewise, when lead is chemically plated onto cadmium in accordance with the present procedure, there is formed a lead-cadmium alloy which melts below 280 C. The bath in such case can be operated at a temperature of about 280 C., although the melting point of cadmium is 321 C. and the melting point of lead is 327 C.

Once the base metal article is immersed in the bath at operating temperature molten metal begins to form thereon. ;The base metal article is left in the bath until the desired amount of coating has been so formed. Then.

the coated article is removed from the bath .and the molten metal is permitted to solidify. The coated article may then be subjected to cleaning and finishing operations, as desired, such as rinsing, etching, and the like.

I Agitation is advantageous during the procedure since it helps to provide a reproducible rate of chemical plating as local heating of the immersed metal due to heat evolved during the reaction is avoided. Agitation also assures that the concentration of ions in the vicinity of the immersed metal does not change as the reaction proceeds.

The process of the present invention will be more readily understood from a consideration of the following specific examples which are given for the purpose of illustration only and are not intended to limit the scope of the invention in any way.

Example I Example 11 A saturated (at 25 C.) solution of indium trichloride in glycerol is heated to 180 C. Pieces of zinc metal immersed in this bath as in Example I are quickly plated with a smooth, uniform coating of molten indium metal containing a small amount of zinc which solidifies upon removal of the coated object from the bath.

Example 111 Stannous chloride is melted and maintained at a temperature of 300 C. Pieces of zinc metal immersed in the molten stannous chloride at this temperature rapidly become covered with a smooth, uniform coating of molten tin containing a small amount of zinc. Upon the removal of the coated zinc objects from the bath, the coating solidifies and the assemblies are washed with water.

Example IV A mixture of ten grams of zinc chloride and ten grams of stannous chloride are maintained at a temperature of 300 C. Pieces of zinc metal immersed in this bath are coated with molten tin containing a small amount of zinc, but at a substantially slower rate, about 80 mils per minute, than is the case with Example III.

Example V Stannous chloride is melted and maintained at a temperature of 300 C. Cadmium metal immersed in the molten stannous chloride is rapidly covered with a smooth, uniform coating of molten tin containing a small amount of cadmium at an estimated plating rate of about.

30 mils per minute.

Example VI One gram of indium trichloride is added to grams of glycerol and the solution heated at 180 C. Cadmium immersed in this bath is provided with a smooth, uniform coating of molten indium containing a small amount of cadmium at an estimated plating rate of about 5 mils per minute.

' Example VII One gram of stannous chloride is added to 10 grams of glycerol and the solution heated at 200 C. Cadmium immersed in this bath is provided with a molten coating of cadmium-tin alloy.

Example VIII One gram of plumbous bromide is added to 10 grams of a zinc chloride-ammonium chloride eutectic (3 to 1, by weight, melting point 180 C.) and heated at 280 C. Cadmium immersed in this bath is provided with a smooth, uniform coating of cadmium-lead alloy.

6 Example IX One gram of indium trichloride is added to 10 grams of glycerol and the solution heated to 220 C. Manganese immersed in this bath is provided with a smooth, uniform coating of molten indium containing a small amount of manganese at an estimated plating rate of about 0.5 mil per minute.

Example X One gram of indium trichloride is added to 10 grams of a molten lithium chloride-potassium chloride eutectic (5:6, by weight, melting point 352 C.) and the solution heated at a temperature of 420 C. Manganese immersed in this bath is provided with a smooth, uniform coating of indium containing a small amount of manganese at an estimated plating rate of about 1 mil per minute.

Example XI One gram of cadmium chloride is added to 10 grams of zinc chloride and the mixture heated at 350 C. Zinc immersed in this bath is coated with molten cadmium containing a small amount of zinc.

Considerable modification is possible in the selection of the combination of base metal and plating metal and the other constituents of the bath as well as in the exact techniques followed without departing from the scope of the invention.

I claim:

1. The method of coating articles comprising at least one of the metals selected from the group consisting of zinc, cadmium and manganese with a lower melting metal which comprises immersing said article in a solution of salt of metal other than the selected base metal and more noble than the selected base metal in a polar organic compound until the desired amount of more noble metal is deposited on the article in a coating through displacement of base metal in the form of an alloy with said base metal, said solution being at a temperature below the solidus point of the base metal but above the liquidus point of the metal coating.

2. The method of claim 1 wherein the organic compound is a polyhydric alcohol.

3. The method of displacing a base metal comprising at least one of the metals selected from the group consisting of zinc, cadmium and manganese with a lower melting metal which comprises immersing said base metal in a liquid, non-aqueous bath comprising salt of metal other than the selected base metal and more noble than the selected base metal until at least a portion of said base metal has been displaced by said more noble metal forming an alloy therewith in molten form, said bath being at a temperature below the solidus point of the base metal and below the solidus point of said more noble metal but above the liquidus point of said alloy.

4. The method of claim 3 wherein substantially all of said base metal is displaced.

5. The method of claim 3 wherein said liquid nonaqueous bath is a molten salt bath.

6. The method of claim 5 wherein said molten salt bath comprises, in addition to salt of the said more noble metal, other salt selected from the group consisting of ammonium salts and salts of metals no more noble than the selected base metal.

7. The method of claim 5 wherein the molten salt bath consists essentially of salt of the said more noble. metal.

References Cited in the file of this patent UNITED STATES PATENTS Gebhardt Jan. 26, 1954 OTHER REFERENCES 

3. THE METHOD OF DISPLACING A BASE METAL COMPRISING AT LEAST ONE OF THE METALS SELECTED FROM THE GROUP CONSISTING OF ZINC,CADMIUM AND MANGANESE WITH A LOWER MELTING METAL WHICH COMPRISES IMMERSING SAID BASE METAL IN A LIQUID, NON-AQUEOUS BATH METAL AND MORE NOBLE THAN OTHER THAN THE SELECTED BASE METAL AND MORE NOBLE THAN THE SELECTED BASE METAL UNTIL AT LEAST A PROTION OF SAID BASE METAL HAS BEEN DSIPLACED BY SAID MORE NOBLE METAL HORMING AN ALLOY THEREWITH IN MOLTEN FORM, SAID BATH BEING AT A TEMPERTURE BELOW THE SOLIDUS POINT OF THE BASE METAL AND BELOW THE SOLIDUS POINT OF SAID MORE NOBLE METAL BUT ABOVE THE LIQUIDUS POINT OF SAID ALLOY. 